Process for the recognition and separation of useful and interfering echoes in the received signals of distance sensors which operate in accordance with the pulse-echo principle

ABSTRACT

In a first step, an individual echo is associated with each local maxima in the received ultrasonic signal. Subsequently, characteristic features such as time position, amplitude and form factor of each respective individual echo are detected numerically for the individual echo. In a second step, the mentioned features are subjected to a fuzzy evaluation. The features form the input variables of the evaluation. A multiple echo probability for each individual echo is available as the result. With this awareness, with the aid of the a priori knowledge and with the aid of the awareness of the history, it is possible, for example, to draw conclusions concerning the filling level of the bulk material in containers in which the surface of the bulk material is at times masked for the ultrasonic beam by the stirring apparatus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to ultrasonic distance sensors based on thepulse-echo process with increased measurement certainty and improvedsuppression of interfering echo signals. Significant areas ofapplication are non-contact distance measurement for the positioning ofworkpieces, collision protection or filling-level metrology.

2. Description of the Prior Art

Ultrasonic sensors are known which determine the distance between thesensor and a sound-reflecting object by measuring the transit time of asound signal from the sensor to the object and back. In this case, theecho is usually detected in that the exceeding of a prescribed thresholdvalue in the received signal is evaluated. This process for distancemeasurement usually evaluates the transit time of the first detectedecho. Any possibly following echoes from other objects situated withinthe detection range of the sensor are not, in contrast, furtherprocessed. By means of time-window control according to Magori, V.;Walker, H.: Ultrasonic presence Sensors with wide range and high localresolution. IEEE Trans. Ultrasonics, Feroelectrics and FrequencyControl, UFFC-34, No. 2, Mar. 1987, p. 202-211, the permissibledetection range for echo signals can in this case be varied in thedesired manner. In this manner, echoes from objects at differingdistances from the sensor can also be detected, in that the evaluationtime window is cyclically displaced over the measurement range; in thiscase, the resolution of the individual echoes and the total duration ofmeasurement increase as the length of the time window decreases.

Processes for the processing of ultrasonic echo signals are moreoverknown in which the received signal is digitally sampled and stored in amemory; in this case, the received signal can also be the demodulatedenvelope curve of the echoes (European Application 0 459 336). Thesignal processing takes place following the recording of the receivedsignal by extraction of the echoes by means of a suitable process, e.g.matched filter+threshold value detection. In this manner, all echoesoccurring within one measurement can be detected.

Further, in the process which is described in Advances inInstrumentation and Control, Vol. 46, part 2, 1991, Research TrianglePark, NC, US, Duncan: "Ultrasonics in Solids Level Measurements", pages1355-1366, the emitted and subsequently received ultrasonic signal isdigitized by means of a microprocessor and stored as an envelope curve.The process relates to the measurement of the filling level of acontainer., To determine the useful echo, the echo profile of the emptycontainer is compared with the echo profile of the filled container.Furthermore, it is possible to recognize the useful echo in that apriori knowledge of the nature of the filling material situated in thecontainer and the echo characteristic thereof are used for therecognition of the useful echo.

Furthermore, processes for the suppression of undesired echoes containedin the received signal, for example due to interfering objects which, inaddition to the measured object, are situated within the detection rangeof the sensor, are known. If the interfering objects are spatially fixedand at the same time the range of movement of the measured object isrestricted, then an adequate suppression of interfering echoes can beachieved by appropriate selection of the evaluation time window.

Furthermore, it is known that interfering object echoes can besuppressed in that, in a learning phase in which the measured object isnot situated within the pickup range of the sensor, in the firstinstance all interfering object echoes are detected and filed in amemory (German OS 33 37 690). During the measurement operation, thecurrently detected echoes are compared with the learned echoes. In theevent of an adequate concordance, the echo is classified as aninterfering object echo and appropriately suppressed, while theremaining echoes are associated with measured objects.

In German OS 33 37 690 and European Application 0 459 336, processes aredescribed which mask out interfering echoes caused by multiplereflections between the sensor and an object in that the maximum transittime to be evaluated is limited, so that echoes occurring outside thistransit time are disregarded. In the case of the solution presented inEuropean Application 0 459 336, the echo amplitude can additionally alsobe evaluated as criterion for the multiple echo suppression. However,these processes are in general unsuitable for measurement situationsinvolving a number of objects in the pickup range of the sensor.

Furthermore, processes are known for the suppression of interferingechoes in the basis of plausibility checks (German OS 38 20 103 andGerman OS 38 21 577). Since the extent which the measurement situationcan change is limited due to the finite speed of movement of objects,echoes are evaluated only when their time position and amplitude aresufficiently plausible on the basis of their extent of deviation fromprevious measurement situations. In this manner, interfering signalswhich occur stochastically, in particular can be reliably suppressed.

It is common to all above known processes for the evaluation of echosignals in the case of ultrasonic distance sensors that an object isassociated with each echo, which is detected in the received signal andwhich is not a stochastic interfering signal, within the maximum transittime to be evaluated; in this case, the distance from the sensor isobtained from the sound transit time of the echo. A disadvantage ofthese known processes is that, on this basis, echoes which arise forexample as a result of multiple reflections between the sensor and anindividual measured object and which do not lie outside the maximumtransit time to be evaluated are thus also associated, erroneously, withfurther, actually non-existent objects. This may lead to very greaterrors in the assessment of measurement situations, especially whenmeasured objects are situated at a short distance from the sensor.

The sound signal may be reflected repeatedly between the acoustictransducer and the objects which are situated within the pickup range ofthe sensor. As a function of the distance between the object and thesensor, the object reflectivity and the geometry of the acoustictransducer, as well as of the propagation attenuation, these multipleechoes decay more or less rapidly. When using a planar transducersurface or reflector surface and in the case of a short distance betweenthe transducer and the object, the decay time constant of the multipleechoes is in the order of magnitude of the single sound transit time.The latter is obtained from the path from the sensor to the object andback. As a result, a plurality of echoes of the same object are detectedin the received signal. Additional interfering echoes may occur where aplurality of objects are situated within the pickup range of the sensor.This is caused by reflection paths between the individual objects ormultiple reflections on various objects.

In the case of all known processes for echo-signal processing, theproblem exists that the interfering echoes arising as a result ofmultiple reflections are not distinguished from the direct objectechoes; this leads to erroneous measurements in the case of manysituations occurring in practice.

SUMMARY OF THE INVENTION

It is an object of the preceding invention to specify a process whicheliminates the above described interfering echoes.

The above object is achieved in accordance with the principles of thepresent invention in a process for the recognition and separation ofuseful and interfering echoes in the received signal of pulse-echodistance sensors, wherein the maxima in the received signal are detectedand an echo is associated with each maximum, form factors whichcharacterize the form of the echoes are produced and stored, theamplitudes and the times of occurrence of the maxima are measured andstored, differences between the measured amplitudes and expectedamplitude values are determined, differences between the measured timesof occurrence of the maxima and expected values for such occurrencetimes are determined, differences between the form factors and expectedform factor values are determined, the probability that a multiple echohas been received is assumed to be larger as the aforementioneddifferences become smaller, and wherein the probability of the presenceof an interfering echo is assumed to be larger as the probability of thepresence of a multiple echo increases.

The invention can advantageously be used for intelligent distancesensors having object-selective measurement properties, especially fordistance measurement under conditions which are made more difficult byinterfering objects.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the principle of the pulse-echo process.

FIG. 2 illustrates the creation of multiple echoes by reflection of thesound signal at the transducer or between the various objects.

FIG. 3 illustrates the selected features of an individual echo which areanalyzed in accordance with the inventive method.

FIGS. 4a through 4e illustrate a numerical example for explaining theprinciple of multiple echo evaluation in accordance with the inventivemethod.

FIGS. 5a through 5e illustrate the echo profile evaluation in accordancewith the inventive method in the case of a time-varying echo profile,for example, due to the rotary motion of a stirring apparatus.

FIGS. 6a through 6c illustrate the echo profile evaluation in accordancewith the inventive method by comparison of a current echo profile with alearned echo profile.

FIG. 7 illustrates a block diagram of an embodiment of circuitarrangement for filling-level measurement by echo detection as well asfor the extraction of echo features in accordance with the inventivemethod, wherein a microcontroller is employed the control and theevaluation of the measurements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Non-contacting ultrasonic distance measurement is based on thedetermination of the transit time of a sound signal from an acoustictransducer to the measured object and back (pulse-echo process). Theprinciple is shown in FIG. 1. The pulsed emission signal S is emittedperiodically, reflected at the reflector (measured object) and isavailable at the output of the ultrasonic transducer, delayed by thetime t_(e). In most cases, only the envelope curve signal HK of thehigh-frequency received signal is evaluated. Depending upon the selectedsound frequency, the range of measurement can be from a few millimetersto tens of meters. Using ultrasound, a high path resolution in thedirection of the sound propagation can be achieved with relatively lowexpenditure. In contrast to the optical system, an important advantageis the high degree of insensitivity to dust and illumination conditions,as well as to the material condition, color and surface roughness of themeasured objects. The shape and amplitude of the echo signals maydepend, to a great extent, upon the orientation as well as the geometryof an object. This is caused by shadowings, specular reflections andinterference. These act as interfering variables for the purposes ofdistance measurement. A further problem is that in the case of highlyreflective objects, as a consequence of the reflection of the receivedsignal at the transducer, multiple echoes frequently occur, whichsimulate the presence of further objects (see FIG. 2). This can falsifythe result in cases wherein the measurement depends on a determinationof distances other than merely the shortest distance to an object withinthe range of the acoustic signal.

The emission signal S, which is emitted by the ultrasonic transducerUSW, is in the first instance reflected at the object O_(a). E_(a)represents the envelope curve HK of the first useful echo of the objectO_(a). The second echo E_(a'), (first multiple echo) arrives at theultrasonic transducer USW before the second useful echo E_(b), whichoriginates from the object b. E_(a") is the second and E_(a"') the thirdmultiple echo of the object O_(a). E_(b') is the first multiple echo ofthe object O_(b). E_(ab') is the echo which arises due to the parasiticreflection path between the objects O_(a) and O_(b).

The echoes and their respective envelope curves are provided with thefollowing indices, and the composition of their transit times are asfollows:

    ______________________________________                                             Ind                                                                      Echo Ex     Transit time   Meaning                                            ______________________________________                                        E.sub.a                                                                            1      t.sub.e1       basic echo from object a                           E.sub.a'                                                                           2      t.sub.e2 = t.sub.e1 + t.sub.e1                                                               first multiple echo from                                                      object a                                           E.sub.b                                                                            3      t.sub.e3       basic echo from object b                           E.sub.a"                                                                           4      t.sub.e4 = t.sub.e1 + t.sub.e2                                                               second multiple echo from                                                     object a                                           E.sub.ab'                                                                          5      t.sub.e5 = t.sub.e3 + (t.sub.e3 - t.sub.e1)                                                  echo between a and b                               E.sub.a' "                                                                         6      T.sub.e6 = t.sub.e1 + t.sub.e4                                                               third multiple echo from                                                      object a                                           E.sub.b'                                                                           7      t.sub.e7 = t.sub.e3 + t.sub.e3                                                               first multiple echo from                                                      object b                                           ______________________________________                                    

In the case of filling-level measurement, a number of interferingvariables frequently act simultaneously. The echo backscattered at thesurface of bulk material possesses, in general, a substantially loweramplitude than reflections or multiple echoes generated by struts orother structural elements above the bulk material. Added to this aretemporary shadowings due to stirring apparatuses as well as deposits onthe container walls. Further interfering influences are provided by airturbulence and extraneous sound, which may cause amplitude fluctuationsof more than 20 dB.

The object of the signal processing is to separate reliably the echocoming from the surface of the filling material. Since the envelopes ofthe individual echoes in the received signal do not necessarily havesignificant differences, simple recognition by reference to the signalshape is scarcely possible in practice.

Accordingly, the separation of useful and interfering variables takesplace using a fuzzy evaluation unit. In this case, the following act asinput variables:

features for the characterization of an individual echo

features to describe the relations between a plurality of individualechoes

history

a priori knowledge.

The a priori knowledge is always situation-specific. In the case offilling-level measurement, it is assumed that the filling materialproduce the echo which is the furthest distant and which is not amultiple echo. The knowledge of the position of the fixed targets in thecontainer (e.g. in the form of a learned echo profile) and of themaximum filling and emptying rates can also be used as a prioriknowledge. Consideration of the history of the respective profiles ofpreviously-received signals permits plausibility checks concerning thesuppression of erroneous measurements and compensation for driftphenomena.

The echo profile of an individual measurement must be assessed byreference to significant criteria (FIG. 3). The investigations carriedout have shown that the respective echo can be described sufficientlywell by the following features:

time position of the maximum transit time t_(e)

signal amplitude A_(e) of the maximum

form factor Fe (from the 6 dB widths of the maximum).

One recording of the envelope curve is sufficient to simplify theevaluation effort. All features for describing relations between in eachcase two echoes relative to one another are variables derived therefrom.

Each multiple echo is characterized by the fact that it may be derivedfrom one or more preceding echoes. From these preechoes, which also maythemselves be multiple echoes, it is possible to determine expectedvalues for the features of a multiple echo, having regard to the spatialdivergence of the sound signal as well as the frequency-dependentpropagation attenuation.

A useful echo which occurs at time t_(e) usually possesses a 1st-ordermultiple echo at time 2t_(e), a 2nd-order multiple echo at time 3t_(e)etc. Accordingly, the following is applicable for the expected transittime t_(e) of a multiple echo at the position k (in this connection, seealso FIG. 2):

    t.sub.ek =t.sub.ei +t.sub.ej (1<=i<=k, 1<=j<=k)

i,j,k: echo index; a higher index corresponds to a longer transit timei=j: first multiple echo of the object at the position i.

Additional multiple echoes may be generated by reflection paths betweena plurality of individual objects. In the simplest case (thin plates asreflectors), the following is then applicable:

    t.sub.ek =t.sub.ej +(t.sub.ej -t.sub.ei) (i≠j; 1<=i, j<k)

The amplitude of a signal permits statements concerning the losses whichoccur. The amplitude decreases at least by the factor 1/r. In this case,r represents the distance from the ultrasonic transducer. There is, inaddition, an attenuation α of for example 0.015 dB/λ. This decrease maybe even substantially greater, however, and is dependent especially uponthe occurring reflection factors. With regard to the amplitude A_(e) ofa multiple echo at the position k, cf. FIG. 2 E_(a), E_(a'), E_(a"),E_(a'"), which multiple echo is composed of the preechoes i and j, thefollowing condition must accordingly be satisfied on the basis of thedivergence of the sound beam (A_(e) ˜1/t_(e)) and the propagationattenuation, provided that no reflection paths between the variousobjects make a contribution:

    A.sub.ek ·t.sub.ek <min{(A.sub.ei ·t.sub.ei); (A.sub.ej ·t.sub.ej)}(i,j<k)

In the case of multiple echoes due to a reflection path between twoobjects i and j, at least the following is applicable:

    A.sub.ek ·t.sub.ek <max{(A.sub.ei ·t.sub.ei); (A.sub.ej ·t.sub.ej)}(i,j<k)

The signal shape of an echo can be characterized by differing parameterssuch as, for example, gradient and/or decay time or the ratio ofamplitude to width. For practical purposes, a form factor Fe derivedfrom a plurality of signal shape parameters appears to be expedient. Forthe evaluation process developed within the context of this invention,the form factor was determined from the envelope as follows (see FIG.3):

    F.sub.e =(B.sub.a 6 dB)/(B.sub.e /6 dB)

In this equation, B_(a) is the time which elapses between the attainmentof the amplitude maximum A_(e) and the 6dB fall lying to the leftthereof. Be is the time which elapses before the signal has decayed fromthe amplitude maximum A_(e) to the 6 dB fall lying to the right thereof.

In the case of reflections at planar or simple regularly curvedsurfaces, the signal shape remains essentially preserved. In thesecases, multiple echoes possess a similar envelope to the associatedpreechoes i and j:

    F.sub.ek =F.sub.ei,F.sub.ej

The process for the classification of an echo as useful or multiple echois based on the concept that the features of each detected echo arecompared with the expected values computed from the preechoes.Normalized feature differences prove to be particularly suitable forthis purpose.

D_(Mm) =(M_(means) m -M_(exp) m)/M_(exp) m

M_(meas) : measured feature m, where the feature m may be t_(e), A_(e)or F_(e).

M_(exp) : expected value of the feature m

D_(M) m: scaled feature difference

In this specific case, small differences mean that the relevant echocan, with relatively high probability, be allocated to the "multipleecho" class. Due to the above listed factors influencing the soundpropagation, the expected values M_(exp) always represent only estimatedvalues. A binary yes/no decision in multiple echo assessment is thushardly meaningful at all. Accordingly, the difference values D_(Mi) areused as input variables for a fuzzy evaluation unit. FIGS. 4a 4eillustrate the process with reference to a measurement situation and theassociated echo profile, using the example of two partial echoes (E_(a")and E_(e)). FIG. 4a illustrates the actual distance in millimetersbetween an ultrasound transducer USW and objects O_(a), O_(b) and O_(c),and FIG. 4b illustrates the resulting echo profiles. As a result of thedefuzzification, shown for the time difference, the amplitude differenceand the form difference for each of partial echoes E_(a") (FIG. 4c) andE_(e) (FIG. 4d), a value is allocated to each partial echo, which valuedescribes the multiple echo probability P_(MFE). In FIG. 4e, theordinate shows the probability for a useful echo P_(neo), whichprobability is the complement to the multiple echo probability P_(MFE).In FIG. 4c and 4d, 5m signifies a small difference, mad an average, lglarge, n-lo a negative large and p-lg a positive large, v-lg a verylarge, m-sm a moderately small and m-lg a moderately large difference.

Since the speed of movement of objects within the pickup range of thesensor is always limited, the echo profile cannot charge discontinuouslyfrom one measurement cycle to the other. This knowledge is usually usedfor the plausibility assessment of individual echoes. Since, on theother hand, the speed of the change in the situation is onlyinfrequently known precisely and echoes may strongly fluctuate due toair movements or temporary shadowings, processes using fixed thresholdvalues have proved to be suitable only to a limited extent.

In the case of the filling-level sensor presented here, a plausibilitycheck takes place with the aid of fuzzy rules. In this case, eachindividual echo of the current measurement is compared with the echoesof the preceding measurement cycle. A "good" correlation is present whenboth the transit time difference and the difference of the multiple echoprobabilities P_(MFE) are "small". The absolute values for "small","moderate" and "large" transit time differences are obtained, forexample, from the maximum filling speed and measurement rate. Dependingupon the correlation of an echo of the preceding measurement, the valuefor the useful echo probability P_(neo) =1-P_(MFE) of the current echoesis multiplied by a weighting factor derived from the defuzzification. Inthe case of suddenly vanishing echoes (e.g. masking by stirringapparatus) the current profile is supplemented by the corresponding echofrom the preceding measurement with reduced weighting. Accordingly, therespectively last measurement includes the weighted cumulative resultfrom a plurality of preceding measurement cycles. The process isdiagrammatically shown in FIGS. 5a, 5b and 5c for successivemeasurements i, i+1 and i+2, with the corresponding defuzzification forthe echoes identified as 1 and 2 (time difference, P_(neo) differenceand concordance) being respectively shown in FIGS. 5d and 5e.

A basic problem in ultrasonic distance measurement from a plurality ofobjects at the same time is generated in that genuine object echoes maybe masked by multiple or other interfering reflections. In the case offilling-level measurement, the object consists in reliably detecting thesound signal reflected by the filling material; in this case, theamplitude thereof may be very small in comparison with the fixed targetechoes. The process according to invention additionally makes use of theprinciple of comparison between the fixed target echo profile (includingstirring apparatuses which are periodically situated within the pickuprange of the sensor) stored in a learning phase and the signal profilerecorded in the actual measurement operation. In contrast toconventional filling-level sensors, this comparison is likewise carriedout as described below with the aid of fuzzy rules. Echoes with "good"correlation are allocated to fixed targets and are assessed with lowweighting for the further processing. Of the remaining echoes, the echowith the greatest transit time and which possesses a low multiple echoprobability is allocated to the filling material. If all detected echoesshow sufficiently good correlation with the learned profile (e.g. wherethe filling material is situated at the height of a fixed target) thelast echo is evaluated with a high useful echo probability. Theassessment of the deviations between the learned and the measured echoprofile is carried out in a manner similar to that previously described.The process is illustrated in FIG. 6a-6e. Advantages of the unsharpevaluation can be seen in the compensation for drifts and thus possibleadaptive follow-up of the teach-in profile.

In the event of unfavorable reflection conditions at the surface of bulkmaterial, the filling-level echo which is actually of interest may attimes even be situated within the region of the noise limit of themeasuring system. In this case, the detection of the useful echo over aplurality of measurement cycles is often only sporadically possible. Afurther error may be caused by interfering echoes which arise as aresult of parasitic reflection paths between the bulk material andstructural elements of the container, as shown in FIG. 6c. Since theseechoes arrive at the sound receiver later than the filling-level echo,they may be misinterpreted by the evaluation.

To suppress erroneous measured value outputs, in addition to theabove-described fluctuation assessment a plausibility check of themeasured result is accordingly carried out. This check uses as inputvariables the measured values having the highest useful echo probability(P_(NE) max) from the last n measurements as well as the currentlyindicated value. The indicated value is always updated when the newmeasured value is within the tolerance zone predetermined by the maximumfilling or emptying speed; in this case, a sliding average valueformation is carried out to smooth the indicated profile. In the othercase, the indicated value is overwritten only when all measured valuesof the last n cycles lie within this tolerance zone and the measuredvalue for the echoes having the next high useful echo probability liesabove a predetermined threshold. If the measured values scatter in animpermissible fashion, the last valid indicated value is preserved. Thiscondition is at the same time signalled by an error flag via thedisplay. The plausibility check of the indicated value may likewise takeplace with the aid of fuzzy controllers.

The concept of the evaluation process presented permits the stepwisetesting of the individual modules. For test situations involving aplurality of reflectors, the fuzzy sets for the assessment of multipleechoes, temporal fluctuations and concordance with the teach-in-profilecan individually be set up and optimized.

FIG. 7 shows an apparatus for the filling-level measurement and a blockdiagram of the evaluation unit.

An emission pulse is generated by the microcontroller MC atpredetermined time intervals and is passed via the final emission stageSE to the acoustic transducer USW. The emitted sound signal is reflectedby the objects O_(a), O_(b), O_(c), and upon reception by the acoustictransducer USW causes the generation of an echo profile EP. The echoprofile EP is supplied to a preamplifier/envelope curve demodulator V,so as to obtain demodulated echoes. Whenever a predetermined thresholdvalue is exceeded in the echo profile EP, a comparator (symbolized by avariable gain amplifier) delivers a corresponding output signal, theleading edge of which activates a peak detector (symbolized by arectifier) to measure the echo amplitude as well as an integrator(symbolized by a low pass filter) to determine the width of the echo(form factor). When the maximum echo amplitude has been reached, thepeak detector generates a control signal by which the content of thecounter/memory assembly employed for transit time measurement istransferred to the buffer memory. Upon the occurrence of the trailingedge of the comparator signal, the individual features t_(e), A_(e),F_(e) are transferred to the fuzzy evaluation for echo assessment.

The fuzzy evaluation conducts the multiple echo evaluation by theallocation of a multiple echo probability P_(-neo), the fluctuationevaluation by the allocation of a fluctuation probability P ne, thecomparison with the learned echoes (teach-in) by the allocation of aprobability P_(fill) and the plausibility check by the allocation of aplausibility probability P_(meas). The multiple echo evaluation and thefluctuation evaluation are not limited, in their application, only tofilling-level measurement, but the comparison with what has been learnedand the plausibility investigation are specific to the measurement offilling level.

Alternatively, the complete received signal profile for the demodulatedenvelope curve can be sampled by means of an ADC assembly and placed inbuffer memory. In addition to the echo assessment, the featureextraction is then also carried out by the appropriate software.

The investigations concerning the weighting of the echo features for themultiple echo recognition revealed that the transit time possesses thegreatest evidential relevancy. The echo amplitude may be of importanceespecially when multiple and useful echo coincides. The form factorpossesses the lowest weighting. In the case of overlapping echoes withsimilar amplitude, it contributes to the improved assessment of multipleechoes.

As compared with conventional processes, the described fuzzy comparisonbetween teach-in echo profile and measured signal exhibits substantialadvantages, for example when multiple reflections are generated by thebulk material itself.

The evaluation process according to the invention, which has beendescribed herein for ultrasonic sensors, can also advantageously be usedin non-contacting measuring systems which are based on the propagationof electromagnetic waves. These include, for example, pulse radararrangements for object location at relatively long distances ormicrowave sensors for distance measurement. The assessment of the echoamplitudes can optimally be matched by a priori knowledge to theattenuation acting in the respective propagation medium.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

We claim:
 1. A method for the recognition and separation of useful andinterfering echoes in a received signal of a pulse-echo distance sensor,said method comprising the steps of:detecting maxima in said receivedsignal and associating each maxima with an echo; identifying a formfactor with each echo characterizing a form of that echo, and storingthe form factors for the respective echoes; identifying an amplitude anda time of occurrence for each maximum, and storing the respectiveamplitude and time of occurrence for all of said maxima; measuring adifference between the amplitudes of the respective maxima and expectedamplitude values; measuring a difference between the time of occurrenceof each maximum and expected time of occurrence values; measuring adifference between the form factor for each echo and expected formfactor values; assigning a probability to each echo as to whether thatecho comprises a multiple echo by evaluating each of the amplitude, timeof occurrence and form factor differences with a smaller differenceresulting in an increased probability that said echo comprises amultiple echo; and assigning a probability to each echo as to whetherthat echo comprises an interfering echo with the probability that saidecho comprises an interfering echo being greater as the probability ofsaid echo comprising a multiple echo increases.
 2. A method as claimedin claim 1 wherein the step of identifying a form factor for each echocomprises identifying a ratio between a width of each echo measuredbetween first and second points in time at which the amplitude of saidecho assumes a selected value, and a width of said echo between saidfirst point in time and a time at which said echo reaches its maximum.3. A method as claimed in claim 1 wherein the step of assigning aprobability to each echo as to whether that echo comprises a multipleecho comprises assigning said probability as to whether said echocomprises a multiple echo by fuzzy logic.
 4. A method as claimed inclaim 1 wherein the step of assigning a probability to each echo as towhether that echo comprises an interfering echo comprises assigning saidprobability as to whether said echo comprises an interfering echo byfuzzy logic.
 5. A method as claimed in claim 1 comprising the additionalstep of identifying said expected value of the amplitude of a maximumfor each echo as the reciprocal of a signal transit time, said signaltransit time comprising a time elapsing from an emission of a pulse to areception of said echo.
 6. A method as claimed in claim 1 comprising theadditional step of setting said expected value of said form factor asequal to a form factor of a preceding echo.
 7. A method as claimed inclaim 1 comprising the additional step of directing pulses at materialin a container whose level in said container is to be determined, andidentifying a fill status of said container from said received signals.